The present invent ion relates to a brake fluid pressure control device of a simple structure and having the functions of both antilock and traction control.
The simplest antilock device is known as the so-called recirculating type which comprises a wheel brake fluid pressure control valve including a discharge valve provided in the main fluid passage extending from the master cylinder to each wheel brake, a discharged fluid reservoir for temporarily storing the brake fluid discharged from the discharge valve, and a pump or pumping the brake fluid out of the discharged fluid reservoir and returning it to the main fluid passage. If it is desired to add a traction control function to this device, the structure of the entire device can be simplified most effectively by using the circulation pump for the antilock device as a pressure source for traction control.
In order to employ such a structure, brake fluid has to be supplied to the inlet port of the pump. This can be done in several known ways.
These known ways are classified into two types. In one type, a fluid supply passage is provided so as to extend directly from the reservoir for the master cylinder to the pump inlet port. In the other type, the supply passage is provided to branch from the main passage connecting the master cylinder outlet port to each wheel brake. Of these two types, the latter is advantageous in view of easy mounting on a vehicle and the non-necessity of returning redundant brake fluid to the reservoir for the master cylinder through its inlet port.
The latter method, in which the fluid supply passage branches from the main fluid passage, requires a traction control changeover valve for checking fluid flow from the flow return point from the pump toward the master cylinder during traction control and shutoff valve for checking fluid flow from the fluid supply passage toward the discharged fluid reservoir during normal braking (when master cylinder pressure is applied). A solenoid or a pressure-responsive valve is ordinarily used as the shutoff valve.
But, a solenoid valve is not only relatively expensive, it also has a problem in that, if the valve were not returned to its inoperative position as soon as the brake pedal is depressed during traction control, the brake fluid in the master cylinder would flow into the discharged fluid reservoir, thereby making it impossible to get a sufficient brake force by depressing the brake pedal. In this state, the subsequent antilock control function may be lost, too. Thus, it is necessary to increase the reliability of the electric system, including the brake switch. This requires more cost than is apparent.
A pressure-responsive valve, which requires a cylinder/piston assembly for driving the valve body in response to the pressure, is also unsatisfactory in cost.
It is therefore an object of the present invention to provide a simple and low-cost brake fluid pressure control device of the type described above.